Peroxidase labeled antibodies are used to localize tissue antigen. Use of enzyme and a label permits the localization of antigens at both light and electron microscopic levels since the histochemical reaction product of the enzyme may be made visible with both types of microscopes. In order for the method to be useful in ordinary research and clinical laboratories it requires further refinement particularly in the handling of tissue and cells. Development of fixtures suited for immunoelectron microscopy and a study on the penetration of the labeled antibodies into the fixed tissues and cells are the topics requiring immediate attention. Another major problem to be studied is the antigenic differentiations in cell nucleus, particularly that of non- histone chromatin proteins.